First evidence of a black hole's accretion disk found

Astronomers have now detected the clearest evidence of an accretion disk encircling a black hole at the center of galaxy III Zw 002.
Mrigakshi Dixit
Representational image of a black hole.
Representational image of a black hole.


The center of galaxies hosts some of the most chaotic cosmic environments due to the existence of an actively feeding supermassive black hole. 

These cosmic behemoths are surrounded by the spinning accretion disks composed of gas and dust. This glowing disk emits excessive energy over the electromagnetic spectrum, from high-energy gamma rays and X-rays to visible light, infrared, and radio waves.

Astronomers have now discovered the clearest evidence of an accretion disk encircling a black hole at the center of galaxy III Zw 002. As per the official release, the findings were made after reviewing data from "two rare and peculiar near-infrared emission lines."

The observations were made using the Gemini North telescope in Maunakea, Hawai'i, USA. 

The observation of emission lines

Direct observation of the accretion disks using telescopes is challenging due to two factors: their vast distances from the Earth and extremely small sizes. 

As a result, astronomers rely on the light spectrum released from within the disk to determine its size and behavior. 

This methodology has enabled astronomers to make the first-ever detection of the “two near-infrared emission lines” originating from the accretion disk” of galaxy III Zw 002. 

“Emission lines result when an atom in an excited state drops to a lower energy level, releasing light in the process. Since every atom has a unique set of energy levels, the emitted light has a discrete wavelength that acts like a fingerprint identifying its origin,” explained the release. 

The accretion disk presence is particularly inferred through the broad emission lines known as a double-peaked profile. The broad line region is the region of the accretion disk where these lines originate.

“For the first time, the detection of such double-peaked profiles puts firm constraints on the geometry of a region that is otherwise not possible to resolve. And we now have clear evidence of the feeding process and the inner structure of an active galaxy,” said Alberto Rodriguez-Ardila from the Instituto Nacional de Pesquisas Espaciais in Brazil.

First evidence of a black hole's accretion disk found
An artist’s impression of a supermassive black hole with an accretion disk orbiting it. The annotations show a hypothetical double-peaked profile with arrows indicating where in the broad line region each peak originates.

The observations could shed light on the dynamics of the accretion disk

In this case, the two detected emission lines are “Paschen-alpha” and “O I.” 

This observation led to the various measurements of the accretion disk. Upon meticulous analysis, it was found that the “Paschen-alpha” line has an origin radius of 16.77 light-days (the distance light travels in one Earth day from the supermassive black hole). 

While the O I line comes from the accretion disk at a radius of 18.86 light-days. 

The measurements also led to the prediction that the broad line region's outer radius is likely to be roughly 52.43 light-days. 

“The model also indicates that III Zw 002’s broad line region has an inclination angle of 18 degrees with respect to observers on Earth, and the supermassive black hole at its center is 400–900 million times the mass of our Sun.”

The latest observations might aid in a better understanding of the shape and dynamics of the accretion disk. Observing accretion disks could also provide important information about black holes and the development of their host galaxies.

The results have been published in the Astrophysical Journal Letters.

Study abstract:

Double-peaked profiles associated with the broad-line region (BLR) of active galactic nuclei (AGNs) are regarded as the clearest evidence of the presence of an accretion disk. They are most commonly detected by means of optical spectroscopy in the Balmer lines and in the Mg iiλ2798 ultraviolet line. Here, we report the first unambiguous detection of a double-peak broad emission line associated with the O iλ11297 emission line in the near-infrared (NIR) in the local Seyfert 1 galaxy III Zw 002. Additionally, we detect simultaneously in the spectrum the double-peak emission in the Paα line and very likely in the He iλ10830. This is the first time that several broad double-peaked NIR emission lines have been detected simultaneously. The double-peaked profiles are fit using a disk-based model, with an additional Gaussian component attributed to nondisk clouds, which represents the classical BLR. Our results obtained from the fits reveal important parameters, such as disk inclination and geometry. From the double-peaked profile fits, we suggest that the BLR in III Zw 002 has a disk-like geometry, as it extends up to the outer edge of the BLR.

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